Conversion of nitroso compounds to isocyanates



United States Patent "ice 3,467,687 CONVERSION OF NITROSO COMPOUNDS TO ISOCYANATES William Baptist Hardy, Bound Brook, and Robert Putnam Bennett, Bridgewater Township, Somerset, N.J., assignors to American Cyanamid Company, Stamford, Conn., a corporation of Maine No Drawing. Filed July 5, 1966, Ser. No. 562,492 Int; Cl. C07c 119/04 US. Cl. 260-453 10 Claims ABSTRACT OF THE DISCLOSURE Organic isocyanates are prepared by reacting a carbocyclic aromatic nitroso compound, a Lewis acid and carbon monoxide in the presence of a catalyst consisting of a noble metal or compound thereof, this reaction being conducted under anhydrous, hydrogen-free conditions at a pressure above about 1,000 p.s.i. and temperature above about 150 C.

This invention relates to a new method of preparing organic isocyanates. More particularly, it relates to the preparation of isocyanates by reacting an organic nitroso compound and carbon monoxide under elevated temperature and pressure conditions and in the presence of a suitable catalyst.

Tons of isocyanates, particularly aromatic isocyanates, are produced and consumed annually. Commercial needs are currently satisfied by a process which comprises reacting phosgene with an amino compound corresponding to the desired isocyanate, the reaction being conducted at elevated temperatures and pressures. Both phosgene and aromatic amino compounds are relatively expensive materials, and operations are often complicated because of the extreme toxicity of phosgene gas. For these and other reasons, there is a definite need for a new method to serve as a commercial route to isocyanates.

It is an object of this invention to provide a method by which isocyanates can be prepared from different starting materials. It is a further object to provide a method by which isocyanates can be prepared without use of phosgene. Other objects will become apparent from the ensuing description of this invention.

In accordance with this invention, it has been discovered that these objects can be efficiently accomplished by a new process which requires as its essential starting materials an organic nitroso compound, carbon monoxide and a special type of catalyst. The reaction of this invention effects the replacement of the oxygen on the nitroso radical by carbonyl with the consequent formation of an isocyanate. The reaction can be schematically represented as follows:

In the foregoing equation, the symbol R is intended to represent an organic radical and x represents an integer. The foregoing reaction is conducted under substantially anhydrous conditions in a substantially hydrogen-free atmosphere.

The reaction between the nitroso compound and carbon monoxide may be carried out in an autoclave or any other high pressure reactor. A simple procedure is to charge the nitroso compound and catalyst in a solvent, if one is employed, into the reaction vessel, introduce the proper amount of carbon monoxide, and then heat the mixture to obtain the desired reaction pressure and temperature. The reaction can be conducted as a con- 3,467,687 Patented Sept. 16, 1969 tinuous operation or batchwise. Of course, the order of addition of the reactants may be varied to suit the particular apparatus which is employed. For example, the reactants may be introduced on a continuous basis into the heated reactor while, at the same time, the product is withdrawn. The reaction product is recovered and then treated by conventional procedures to effect separation of isocyanate from unreacted starting material, solvent, by-product, etc.

The present invention provides a generally applicable process for converting either monoor poly-nitroso derivatives to the corresponding isocyanates.

Typical of the nitroso compounds which can be converted to isocyanates are carbocyclic aromatic derivatives such as nitrosobenzene, 0-, mand p-dinitrosobenzene, l-nitrosonaphthalene and 1,Z-dinitrosonaphthalene. Likewise, heterocyclic derivatives may also be used.

The process of this invention is applicable to nitroso compounds with or without other substituents, such as alkyl, alkenyl, alkoxy, halogen, acylamido, hydroxy, nitro, mercapto, alkylthio, carboxy, carbalkoxy, cyano, acyl, sulfo, sulfonyl, sulfamyl, carbamyl, phosphono, phosphino and silyl radicals. Among the substituted nitroso compounds useful as starting materials herein, are p-nitrosotoluene, p-nitrosoanisole, p-nitrosophenol, p-nitroso-m-cresol, Z-ethylnitrosobenzene, 4-chloronitrosobenzene, 4-bromonitrosobenzene, 4-fluoronitrosobenzene, p-nitroso N,N dimethylaniline, p-nitrosobenzoic acid, l-nitroso-Z-naphthol, 2-nitroso-p-xylene, 2-fiuoro-4- nitrosotoluene, 1-methoxy-4-nitrosonaphthalene, 3,4-dinitrosoluene, 4-methylthio-l-nitrosobenzene and 4-nitro-1- nitrosobenzene. substituents do not, in general, interfere with the reaction of this invention. Certain substituents may themselves react with carbon monoxide concurrent with the desired reaction, but the latter reaction, nevertheless, occurs. Other groups in the nitroso starting material may react with the isocyanate group, thus yielding derivatives of isocyanates as reaction products. Still others may sterically retard the rate of isocyanate formation without preventing it entirely. ,With these qualifications, the process of this invention is applicable to any organic compound with a nitroso group.

Reaction conditions can be varied over a wide range provided several requirements with respect to pressure and temperature are met. Pressures within the reactor must be in the range of about 40 p.s.i. to 100,000 p.s.i. or higher. Preferably, the pressure should be above 1,000 p.s.i. The reaction will proceed at temperatures above 60 C., and preferably between C., and the temperature of decomposition of either the starting material or the product. The temperature will vary inversely with residence time of material in the reactor. With more highly reactive starting materials, less stringent conditions may be employed. The particular conditions for a given reactant are easily determined in accordance with the foregoing principles.

It is desirable that a solvent be employed when the nitroso compounds are solids under the reaction conditions. Suitable solvents are anhydrous liquids in which the nitroso compound is soluble or dispersible, e.g., benzene, toluene, xylene, aliphatic halogenated hydrocarbons such as 1,1,2-trichloro-1,2,2-trifluoroethane and halogenated aromatic hydrocarbons such as monochlorobenzene, dichlorobenzene and trichlorobenzene. It is preferable that the solvent, as well as the other materials charged into the reactor, be anhydrous, since in the presence of water, isocyanates are converted to urea derivatives.

The amount of carbon monoxide pumped into the reactor should be suflicient to provide at least 2 moles of carbon monoxide per nitroso group. Preferably, however,

a large excess should be employed to give the superatmospheric pressures required for preferred operation of the present invention.

The catalyst for the reaction of this invention comprises a noble metal and a Lewis acid as defined in the reference book by Jack Hine, Physical Organic Chemistry, 1962, McGraw-Hill Book Company, New York. According to the reference, Bronsted acids are included by the term Lewis acids. The noble metal may be used either in a metallic or a chemically combined state. It may be deployed either with or without a physical support. Among the noble metals which can be employed are platinum, palladium, ruthenium, rhodium, osmium, and iridium. Among the chemical forms of these metals which can be used herein are oxides, sulfates, nitrates and halides, as for example: platinum oxide, platinum chloride, platinum nitrate, platinum sulfate and the corresponding palladium compounds.

The Lewis acid component of the catalyst can be a halide (e.g., an iodide, bromide, chloride or fluoride), an acetate, a sulfate or a phosphate of a metal such as tin, titanium, gallium, iron, aluminum or copper.

As specific examples of Lewis acids one can name ferric chloride, ferrous chloride, stannic chloride, stannous chloride, aluminum chloride, titanium tetrachloride, aluminum bromide, gallium trichloride and cupric chloride. Additional examples of the salt type of Lewis acids are listed in the reference book by George A. Olah, Friedel- Crafts and Related Reactions, vol. I, 1963, Int. Publ., New York.

An example of the Bronsted acid type of Lewis acid is anhydrous hydrogen chloride. Other Bronsted acids may be used providing they do not irreversibly react with the isocyanate product. Examples of such reactions are to be found in Recent Advances in Isocyanate Chemistry,

by R. G. Arnold et al., Chemical Reviews 57, 47 (1957).

Within the group of useful Lewis acids, it is preferred to use strong Lewis acids having a halide anion. Chlorides of iron and aluminum are especially preferred.

The physical form of the catalyst can be varied to suit particular needs. The metals can be self-supported or deposited upon a support which disperses the metals so as to increase active surface area. Such porous supports include alumina, silica, carbon, barium sulfate, calcium carbonate, asbestos, bentonite, diatomaceous earth, fullers earth, and the like.

It is possible to deposit the noble metal on a support and form the Lewis acid component in situ by conducting the reaction in a vessel which can supply a cation, when using a solvent medium which can supply an anion under reaction conditions. Similarly a noble metal and a base metal capable of forming a Lewis acid (e.g., iron or aluminum) may be deposited on a porous support. This base metal, in conjunction with a solvent medium comprising a halogenated solvent (e.g., 1,1,2-trichloro- 1,2,2-trifluoroethane or a similar halogenated aliphatic hydrocarbon), forms a Lewis acid under reaction conditions. Other means of forming the Lewis acid in situ will be apparent from these procedures.

A very useful catalytic system consists of palladium, supported on alumina, and ferric chloride. The catalyst should be used in an effective amount. This amount will be determined by reaction pressure and temperature, purity of the nitroso starting material, etc. Once it is known that the desired reaction proceeds in the presence of a noble metal and Lewis acid catalyst, it is within ordinary means to determine how much of each will be used. It has been found that a useful range is in the area of about to 10- mole of noble metal and 5 10 to 5X 10 mole of Lewis acid per mole of nitroso group. Actually, as long as even trace amounts of the metals are present, reaction will proceed. The upper limit of catalyst usage is governed primarily by cost considerations. A preferred catalyst system will have about 0.02-0.00] mole of Lewis acid and 0.05 to 0.005 mole of noble metal per mole of nitroso group. Within these areas, the centers of the respective ranges are especially preferred, but this preferred range depends greatly on the equipment and conditions used, i.e., the amount of agitation, concentrations, temperature, pressure, etc.

This invention is illustrated in the following examples, in which percentages are on a weight basis.

Example 1 NCO A suitable pressure vessel with stainless steel walls is charged with parts of 1,1,2-trichloro-1,2,2-trifiuoroethane, 10.7 parts (0.1 mole) of nitrosobenzene and 5 parts of 5% palladium on alumina. The pressure vessel is sealed and flushed 3 times with carbon monoxide. Car bon monoxide is introduced into the clave until a pressure of 2,800 p.s.i. is obtained. With agitation, the pressure vessel is heated to C. The internal pressure is then about 4,000 p.s.i. After maintaining the temperature at 170 C. for 5 hours, the pressure vessel is cooled to room temperature, vented, flushed with nitrogen and opened. The solvent-soluble material is removed, and the pressure vessel is rinsed with additional solvent. The combined solvents are filtered, and the solvent is removed from the product by distillation under reduced pressure. The crude product is then distilled to give phenyl isocyanate.

Example 2 NCO The general procedure of Example 1 is repeated, substituting an equivalent amount of o-nitrosotoluene for the nitrosobenzene, and using a temperature of C. and pressure of 11,000 p.s.i. The product is o-tolyl isocyanate.

Example 3 NCO The general procedure of Example 1 is repeated, substituting an equivalent amount of p-nitrosotoluene for the nitrosobenzene and a pressure of 13,800 p.s.i. The product is p-tolyl isocyanate.

Example 4 NCO The procedure of Example 1 is repeated, substituting an equivalent amount of 4-chl0ronitrosobenzene for the nitrosobenzene and using a pressure of 13,000 p.s.i. The product contains p-chlorophenyl isocyanate.

Example 5 The procedure of Example 1 is repeated, substituting an equivalent amount of p-nitrosoanisole for the nitrosobenzene. The product is p-methoxyphenyl isocyanate.

Example 6 The procedure of Example 1 is repeated, substituting an equivalent amount of 2-nitroso-p-xylene for the nitrosobenzene. The product is 2,5-dimethylpheny1 isocyanate.

Example 7 NC 0 5 i S 0 Ha The procedure of Example 1 is repeated, substituting an equivalent amount of 4-methylthio-l-nitrosobenzene for the nitrosobenzene. The product is 4-methylthiophenylisocyanate.

Example 8 The procedure of Example 1 is repeated, substituting an equivalent amount of l-nitrosonaphthalene for the nitrosobenzene. The product is l-naphthyl isocyanate.

Examples 9-13 The general procedure of Example 1 is repeated, using different noble metal catalysts, temperatures and pressures shown below. Phenyl isocyanate is obtained.

Tempera- Pressure Noble Metal Catalyst ture C.) (p.s.i.)

5% Pd/BaSO4 170 10,500

5% Pd/CaC0s.. 180 13, 500 5% Ell/alumina 130 11, 500

Pd Black 170 14, 500

Example 14 Example 15 The procedure of Example 14 is repeated using 12.3 parts of nitrosobenzene, 80 parts of benzene, 10 parts of 5% palladium on carbon and 0.44 part of stannic chloride. After carbon monoxide is introduced to a pressure of 4,000 p.s.i., the pressure vessel is heated at 190 C. for five hours. Phenyl isocyanate is obtained.

6 Example 16 The procedure of Example 14 is repeated using 12.3 parts of nitrosobenzene, parts of benzene, 5 parts of rhodium on carbon and 1.41 parts of aluminum bromide. After carbon monoxide is introduced to a pressure of 3,000 p.s.i., the pressure vessel is heated at 190 C. for five hours. Phenyl isocyanate is obtained.

Example 17 The procedure of Example 14 is repeated using 12.3 parts of nitrosobenzene, 80 parts of benzene, 5 parts of rhodium on carbon and 0.5 part of ferrous chloride. After carbon monoxide is introduced to a pressure of 2,700 p.s.i., the pressure vessel is heated at 190 C. for five hours. Phenyl isocyanate is obtained.

Example 18 The procedure of Example 14 is repeated using 12.3 parts of nitrosobenzene, 80 parts of benzene, 5 parts of rhodium on carbon and 0.75 part of aluminum chloride. After carbon monoxide is introduced to a pressure of 3,000 p.s.i., the pressure vessel is heated at 190 C. for five hours. Phenyl isocyanate is obtained.

We claim:

1. A process of preparing an isocyanate which comprises reacting, in the presence of a catalyst consisting essentially of a noble metal or compound thereof and a Lewis acid, said compound being selected from the group consisting of oxides, sulfates, nitrates and halides, carbon monoxide and a carbocyclic aromatic nitroso compound, in amounts of at least 2 moles of carbon monoxide per nitroso group, said reaction being conducted under substantially anhydrous, hydrogen-free, super-atmospheric pressure conditions and at an elevated temperature below that at which the starting materials and the product isocyanate decompose.

2. The process of claim 1 wherein the Lewis acid is a member selected from the group consisting of the halides of iron, aluminum, tin, titanium, gallium and copper.

3. The process of claim 1 wherein the noble metal is dispersed on a porous support.

4. The process of preparing an isocyanate in accordance with claim 1 which comprises, in the presence of a catalyst consisting essentially of a noble metal or compound thereof and a Lewis acid, said compound being selected from the group consisting of oxides, sulfates, nitrates and halides, reacting carbon monoxide and a carbocyclic aromatic nitroso compound, in amounts of at least 2 moles of carbon monoxide per nitroso group, said reaction being conducted under substantially anhydrous hydrogen-free conditions at a pressure of at least 1,000 p.s.i. and at an elevated temperature below that at which the starting materials and the product isocyanate decompose.

5. The process of preparing an isocyanate in accordance with claim 1 which comprises, in the presence of a Lewis acid and noble metal catalyst or compound thereof, said compound being selected from the group consisting of oxides, sulfates, nitrates and halides, reacting carbon monoxide and a carbocyclic aromatic nitroso compound, in amounts of at least 2 moles of carbon monoxide per nitroso group, said reaction being conducted under substantially anhydrous, hydrogen-free conditions at superatmospheric pressure and at a temperature of at least C.

6. The process of claim 1 which comprises, in the presence of a Lewis acid and noble metal catalyst or compound thereof, said compound being selected from the group consisting of oxides, sulfates, nitrates and halides, and an inert anhydrous liquid solvent, reacting carbon monoxide and a carbocyclic aromatic nitroso compound, in amounts of at least 2 moles of carbon monoxide per nitroso group, under substantially anhydrous, hydrogenfree conditions, said reaction being conducted at a pres- 7 sure above about 1,000 p.s.i., and at a temperature above about 150 C., but below that at which the starting materials and the product isocyanate decompose.

7. The process of claim 6 wherein the Lewis acid is a member selected from the group consisting of the halides of iron, aluminum, tin, titanium, gallium and copper.

8, The process of claim 7 wherein the noble metal is dispersed on a porous support.

9. The process of claim 1 wherein the nitroso compound is nitrosobenzene.

10. The process of claim 1 wherein the noble metal 8 References Cited UNITED STATES PATENTS 3,063,980 11/1962 Bloom et a1. 260-689 XR 3,070,618 12/1962 Drummond 260-453 3,370,078 2/1968 Bennett et a]. 260-453 FOREIGN PATENTS 1,025,436 4/1966 Great Britain.

10 CHARLES B. PARKER, Primary Examiner D. H. TORRENCE, Assistant Examiner US. Cl. X.R.

is platinum, palladium, ruthenium, rhodium, osmium or 15 252-460, 472; 260-465, 469, 471, 515, 556, 558, 606.5,

iridium. 

